Profiles made of thermoplastics are manufactured by the profile extrusion process. The thermoplastic material is prepared into a melt in an extruder and initially shaped in a die. The profile is given its final contours in the downstream sizing section, where it is cooled at the same time. A typical extrusion plant includes an extruder, a sizing table, a take-off unit and an automatic saw. Other stations can also be added, such as a delivery table, a punching or printing facility and other special processing units.
When using a profile extrusion process, a manufacturer usually desires an elastomer that “shear thins” or decreases in viscosity with applied shear forces. Because pressure drop across an extruder die and amperage required to turn an extruder screw are directly related to elastomer viscosity, a reduction in elastomer viscosity due to shear thinning necessarily leads to a lower pressure drop and a lower amperage requirement. The manufacturer can then increase extruder screw speed until reaching a limit imposed by amperage or pressure drop. The increased screw speed translates to an increase in extruder output. An increase in shear thinning also delays onset of surface melt fracture, a phenomenon that otherwise limits extruder output. Surface melt fracture is usually considered a quality defect and manufacturers typically limit extruder output and suffer a productivity loss to reach a rate of production that substantially eliminates surface melt fracture.
When producing profile extrusions with thin walls and a complex geometry, a manufacturer looks for an elastomer with high melt strength (“MS”) and rapid solidification upon cooling in addition to good shear thinning behavior. A combination of a high MS and rapid solidification upon cooling allows a part to be extruded hot and cooled below the elastomer's solidification temperature before gravity and extrusion forces lead to shape distortion. Ultimately, for broad market acceptance, a finished part should also retain its shape despite short term exposure to an elevated temperature during processing, shipping or eventual use.
Gaskets are used in a variety of applications, for example in appliances such as refrigerators and freezers requiring a flexible gasket for sealing the area between the door and appliance body. One of the most commonly used materials for the production of gaskets is polyvinyl chloride (“PVC”). However, PVC requires compounding and formulating in order to incorporate the various additives necessary for imparting desirable properties to the gaskets. Besides the additional time and money required for the additional mixing steps, additives such as plasticisers can absorb spills and become discolored. Plasticiser additives are also susceptible to attack by microbes which can also lead to discoloration of the gasket, e.g., black stains. Furthermore, PVC gaskets become brittle at low temperatures and cracking becomes a problem. Thus at low temperatures, PVC gaskets are difficult to install. PVC gaskets are also perceived as having a negative impact on the environment.
Therefore, there is a need for a material for profiles and gaskets that would provide increase extrusion line speeds (i.e., lower torque and lower die pressure) and improved performance in terms of reduced melt fracture, collapse resistance, etc.